Two charged dust particles exert a force of 8.8 10-2 N on each other. What will be the force if they are moved so they are only one-fourth as far apart?
Here, F=kq1q2/d^2
but the new distance is 1/4 of original d.
So new Force between the particles will be 8.8*16* 10^-2 = 1.408 N
Hope you understand this.
Good Luck !